How to make semitransparency on an image plane of a 2D png in v2.8?
Of course I figure it out right after asking, after a day of no progress.

To help the next person out:


Import your Image Plane
In Materials -> Settings -> Blend Mode and Shadow Mode select "Alpha Blend", this displays existing image transparency
Still in Materials -> Settings check Show Backface (important!)
In Materials -> Surface, add a new Shader, you can do this by clicking the white circle on the last existing shader, and select Principled BSDF
Switch to the Shading workspace, at the very top bar next to the Rendering tab you can access it from General under the + sign.
The Principled BSDF will already have an output to the Mix Shader. You need to connect the Colour output from the PNG to the Base Colour of the Principled BSDF.


Now you can modify the alpha in the Principled BSDF and it will show up in the image plane.

Hope this helps. This is not clear at all for beginners.